Structures and stability of medium silicon clusters.II.Ab initio molecul orbital calculations of Si_(12)-Si_(20)

摘要

Ab initio all-electron molecular-orbital calculations are carried out to study the structures and relative stability of low-energy silicon clusters (Si_n,n=12-20).Selected geometric isomers include those predicted by Ho et al [Nature (London) 392,582 (1998)] based on an unbiased search with tight-binding/genetic algorithm,as well as those found by Rata et al.[Phys.Rev.Lett.85,546 (2000)] based on density-functional tight-binding/single-parent evolution algorithm.These geometric isomers are optimized at the M011er-Plesset (MP2) MP2/6-31G(d) level.The single-point energy at the coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] CCSD(T)/6-31G(d) level for several low-lying isomers are further computed.Harmonic vibrational frequency analysis at the MP2/6-31G(J) level of theory is also undertaken to assure that the optimized geometries are stable.For Si_(12)-Si_(17) and Si_(19) the isomer with the lowest-energy at the CCSD(T)/6-31G(d) level is the same as that predicted by Ho et al.,whereas for Si_(18) and Si_(20),the same as predicted by Rata et al.However,for Si_(14) and Si_(15),the vibrational frequency analysis indicates that the isomer with the lowest CCSD(T)/6-31G(d) single-point energy gives rise to imaginary frequencies.Small structural perturbation onto the Si_(14) and Si_(15) isomers can remove the imaginary frequencies and results in new isomers with slightly lower MP2/6-31G(J) energy;however the new isomers have a higher single-point energy at the CCSD(T)/6-31G(J) level.For most Si_n (n=12- 18,20) the low-lying isomers are prolate in shape,whereas for Si_(19) a spherical-like isomer is slightly lower in energy at the CCSD(T)/6-31G(d) level than low-lying prolate isomers.